Electric snap switch



t. 3, 1940. I E. J. SCHAEFER 2,213,890

ELECTRI C SNAP SWITCH Original Filed July 1, 1937 Fig. I.

. Inventor: Edward J. Schaefer b9 )9 Hi Attorney.

Patented Sept. 3, 1940 UNITED STATES ELECTRIC SNAP SWITCH Edward J.Schaefer, Fort Wayne, Ind., assignor to General Electric Company, acorporation of New York Original application July 1, 1937, Serial No.

151,444. Divided and this application September 29, 1938, Serial No.232,417

8 Claims.

My invention relates to electric snap switches, more particularly tothermally operated snap switches and has for its object a simple,reliable and inexpensive snap switch of this character.

This application is a division of my copending application, Serial No.151,444, filed August 1, 1937, for El ctric protective systems,particularly as applied to electric motors.

My invention is especially useful in the protection of small electricmotors of fractional horsepower size against abnormally hightemperatures by reason of an excessive current flowing therethrough.When used in the protection of a motor, it is contemplated that theswitch will'be mounted on the motor.

In carrying out my invention in one form, I provide a bimetallicthermostatic element for operating a switch contact in response tochanges in temperature togetherwith a special permanent magnet forcausing the movable switch contact to move between the open and closedcircuit positions with a snap action. The permanent magnet is made of amaterial having high coercive force,the magnet itself having relativelysmall physical dimensions whereby it may be used in a switchhavingrelatively small physical dimensions. The permanent magnet also isformed as a block having great compactness and which can be very readilysecured to a supporting member in position to influence the action ofthe movable switch contact.

Another feature of the permanent magnet is its magnetization with anorth pole and a south pole on one face of the block, a groove in thisface being provided between the' two poles for the purpose ofmagnetically isolating the poles and also to provide for convenience insecuring the magnet in place. Preferably, the magnet is mounted on asupporting strap made from a nonmagnetic material such as brass. Thisstrap is provided with a depression forming a seat for the magnet andwith a projection extending into the groove in the magnet to hold themagnet in place.

In one form of my invention, the groove between the poles of the magnetis filled with a suitable circuit making and breaking contact materialsuch as silver with which a suitable movable switch contact cooperates.This arrangement utilizes the magnet to give an arc blowout effect aswell as a snap action in the movement of one of the contacts.

For a more complete understanding of my invention reference should behad to the accompanying drawing. Fig. l of which is a fragmentaryvertical sectional view of an electric motor equipped with a protectivedevice embodying my invention; Fig. 2 is a View partly in section takenalong the line 2-2 of Fig. 1 looking in the direction of the arrows;Fig. 3 is a sectional view of the protective device illustrated in Figs.1 and 2; Fig. 4. is a fragmentary sectional view similar to Fig. 3 butshowing a modified form oi my invention having provision for manualreset; while Fig. 5 is an enlarged elevation view of a modified form ofcontact structure.

Referring to Figs. 1, 2 and 3 of the drawing, I have shown my inventionin one form as applied to a typical small motor of the split phase typecomprising a stator winding l and a squirrel cage rotor H. The snapswitch protective device i2 is mounted in close proximity to the winding10, preferably on the end shield l3 of the motor closely adjacent theprojecting end turns M of the winding. In order to support it inposition, the end flange is constructed with a chamber l bounded by aflange 46 for the operatingmechanism of the protective device.

It will be understood that a split phase motor is provided with' atleast two running windings and a starting winding. These windingshowever are interspersed with one another and form a physically unitarywinding i l the temperature of which reflects the current value in anyparticular winding. Thus the winding I l may in fact include one or moreelectrically separate windings depending upon the type of motor used.

The protective device comprises a supporting plate 11, forming a base,made ,of a suitable insulating material such as a molded phenoliccondensation product or fiber, this plate being of irregular shape, asshown. The thermal current responsive control mechanism is mounted onone side of the plate so as to fit into the chamber when the plate issecured to the end shield over the chamber, it being held in place bysuitable screws I8 and 19.

The mechanism comprises a flexible spring switch arm secured by rivets2! to the plate I! and. having on its free end a contact 22. To providefreedom of movement for this contact arm and also to provide for thecirculation of the air entrapped in the motor housing or casing, anelongated aperture 23 is provided in the plate coextensive with the freeportion of the contact arm and somewhat larger in outline. A secondmovable contact 24 is supported on the end of a bimetallic thermostaticstrip 25 in position to engage the contact 22, the opposite end of thethermostat being secured to the plate ll by a rivet 26. It will beobserved that under normal conditions, when the contacts are inengagement, as indicated in the drawing, the flexible contact arm 20 isbent into the aperture-23 by the pressure exerted upon it by thethermostat. The rivets 2i and 2B constitute terminal 'connections forconnecting the contacts in the motor circuit, the circuit leading fromthe rivets 2| through the contact arm 20, the contacts, the bimetallicthermostat 25 and out through the rivet 26.

For the purpose of providing a snap movement, a permanent magnet 21 isprovided adjacent the end of the thermostat. This magnet is a block ofmetal, specifically a small disk or button, having a central groove 28in one face and suitably magnetized so as to have a north pole and asouth pole on this face which is adjacent the thermostat. These polesare defined by the projections or faces on each side of the groove 28,

the groove improving the magnetic performance 'drawn into engagementwith the face of the magnet when the thermostat bends a predeterminedamount in response to an increase in temperature. The armature 29 isarranged at such an angle on the end of the thermostat as to compensatefor curvature of the thermostat when heated and thereby lie fiat againstthe face of the magnet. I

In order to obtain suificient magnetic force for effective snap actionwhile at the same time limiting the size of the magnet to practicallysmall proportions, I use as a material for the magnet an alloy having ahigh coercive force. Preferably I use that alloy consisting of 12%aluminum, 25% nickel, and copper, the balance being mainly iron, such asdescribed and claimed in Patent 1,947,274, issued February 13, 1934, toWilliam E. Ruder and Patent 2,027,997, issued January 14, 1936, toTokushichi Mishima.

The disk-shaped magnet 21 is secured in position by a metallicsupporting strap 30 which extends over the thermostat in parallel spacedrelation therewith, one end being held by the rivet 26 and the other endhaving a tongue 3| lying in a slot provided for it in the plate I? atthe end of the aperture 23 and being bent over against the opposite sideof the plate I7. A special mounting is provided for the magnet on thestrap 30. This comprises a shallow depression 30a forming a seat for themagnet, which may be struck into the strap 30 by means of a die, and aprojection or tab 32 which is struck inward from the metal of the strapand lies in the slot 28. The tab is preferably struck in place by asuitable die and the magnet is simply, slipped in place, there beingsufficient resiliency in the parts so that the magnet is held firmly inplace. The supporting strap .30 is preferably made of a suitablenon-magnetizable material, such as brass, for the purpose of preventingany interference with the flux distribution of the magnet.

For the purpose of providing for temperature adjustment of thethermostat a projection 33 is provided on the thermostat adjacent therivet 26, this projection being extruded by a punch or die. Cooperatingwith this projection 33 is a cam 34 secured in place by the rivet 26 androtatable about the rivet, although frictionally held quite firmly bythe rivet. This cam is eccentric with respect to the rivet and may berotated into engagement with the projection 33 so as to adjust theposition of the free end of the thermostat under normal temperature.conditions and in so doing adjust the temperature or current setting ofthe thermostat. For example, if the current setting is to be increased,the cam 34 is rotated to depress the free end of the thermostat wherebya greater deflection upward of the thermostat and hence a higher currentis required for the thermostat to move its contact out of engagementwith the contact 22.

The electrical resistance of the thermostat is determined mainly by therequirements of stalled rotor conditions. Preferably, the motor whenstalled should not be energized continuously for a period of time longenough for the maximum permissible temperature of the winding to bereached. This is because of the fact that certain parts of the motor,such as the capacitor in a capacitor type motor, are designed forintermittent duty and may be damaged before the winding reached itsmaximum permissible temperature in one continuous energized or onperiod. Moreover, a temporarily tight or frozen condition of theapparatus which stalls the motor and prevents it from starting is moreapt to be overcome by a large number of short energized periods than bya small number of long energized periods. Therefore, the resistance ofthe thermostat is so selected that under stalled rotor conditions, it isheated by the stalled motor current passing through it at a much morerapid rate than the motor winding. Thus, the thermostat when the motoris first connected to the line under stalled conditions has relativelyshort and substantially equal energized and deenergized periods. Inother words, the thermostat is heated quickly by the motor currentpassing through it and deenergizes the motor in a relatively shortperiod of time. And furthermore, the thermostat cools quickly in arelatively short period of time and again energizes the motor.

As this on and off operation of the thermostat continues, the motorbeing now assumed to be continuously stalled, the on periods becomegradually shorter and the off periods longer as the temperature of themotor increases. This is due to the increasing temperature of the motorand hence increasing ambient of the thermostat whereby the thermostat isheated to its operating temperature more quickly but requires a longerperiod in which to cool sufiiciently to reclose the motor circuit.Finally under these stalled conditions, assuming that the motor isunable to start the device to which it is con nected, an equilibriumcondition will be reached under which the motor is deenergized nearlyall the time, the thermostat closing the motor circuit for a very shortperiod. Under these conditions the motor winding temperature fluctuatesfrom a low temperature near but somewhat higher than the reclosingtemperature of the thermostat, because the thermosat cools more quicklythan the winding, and a high temperature depending upon the length ofthe final on period which is a function of the resistance and thermalinertia of the thermostat. Thus, for a relatively high resistancethermostat, the final on time may be, for example, three to fiveseconds, and the high temperature of the motor winding under stalledconditions will be a few degrees higher than the low temperature butlower than the maximum permissible temperature. I prefer thisarrangement for capacitor motors in order to give better protection forthe capacitor. In a typical device the reclosing temperature wasapproximately 75 C. andthe opening temperature 105 C.

Moreover, under stalled conditions the thermostat necessarily receivessome heat directly from the coil l4, heat being radiated from the coilto the base I! and transmitted from the base to the thermostat byconduction and radiation.

The maximum current that can flow continuously in the winding I4 understalled conditions without damage to the winding is considerably lowerthan the maximum permissible continuous current under runningconditions. This is because of the higher rate of heat dissipation fromthe winding when the motor is running due to the agitation of the airentrapped in the tight motor housing whereby heat is carried from theWinding to the housing for dissipation to the outside. As a result Ihave found that ii the thermostat is adjusted properly to protect themotor as previously described under stalled conditions, it operates todeenergize the motor under running conditions in response to a currentwhich is less than the maximum permissible continuous current in thewinding. This is an undesirable condition because for maximum output ofthe motor it should be permitted to operate up to the maximum current.Therefore, to obtain the maximum operating capacity from the motor underrunning conditions the current adjustment of the thermostat must beraised for the running condition.

This increase in the current adjustment is effected in accordance withmy invention by providing for access to the thermostat of the airentrapped in the motor housing. To that end I provide an opening or slot35 in the lower side of the flange l6 which provides an opening, whenthe protective device is mounted in place, for the air when agitated byoperation of the motor to pass into the chamber l5 and out through theaperture 23, as indicated by the arrows 35. The air is at a temperaturesomewhat lower than the temperature of the winding I l whereas thethermostat is so constructed that it must be heated to a temperaturesubstantially equal to the temperature of the coil H3 in order todeenergize the motor.

Under motor running conditions the heating of the thermostat resultsmainly from the transfer of heat from the motor winding to thethermostat. amount of heating in the thermostat itself by reason of thecurrent passing through it. In still air, i. e., the stalled condition,this current value may cause a temperature rise of T degrees in thethermostat above the surrounding air. In moving air, as when the motoris running, the tempera-ture rise of the thermostat due to this samecurrent may be only above the temperature of the moving air due to thebetter heat transfer of the moving air. Therefore, the temperature ofthe thermostat with the motor running is the temperature of the movingair plus thermostat temperature of 105 C. Thus the.

thermostat was heated to the same temperature as the winding and, sinceit was so calibrated, it

opened the switch at that maximum temperature of 105 C.

There will of course be a certain If the current were 20% lower, thetemperature rise in the thermostat due to the current alone would beapproximately 10 C. instead of 15 C., and it might be expected that themotor winding would have to be heated to a temperature 5% higher beforethe thermostat opened the switch. In other words, it might be expectedthat the thermostat would have to be heated by the circulating air to 95C. which with the 10% rise due to current would cause the thermostat tooperate. This is not the case however because the lower the current thelower the rate of temperature rise of the winding and therefore thelower the temperature differential between the winding and the entrappedair. In other words, the thermostat is heated by the air more nearly tothe tempera-ture of the winding. This tends to compensate for the lowertemperature rise in the thermostat itself because of the lower current,with the result that the thermostat opens the motor circuit on lowercurrents at very nearly the same winding temperature as for the heaviercurrents.

In its operation the thermostat bends sufficiently inresponse to anincrease in tempera-ture, to bring the armature 29 to a predeterminedspaced relation with the magnet at which position the armature issnapped into engagement with the poles of the magnet thereby opening thecircuit with a snap action. The contact 22 is caused to follow thecontact z l during this preliminary movement of the thermostat by reasonof the straightening of the contact arm 29. The motor and the protectivedevice then cool and at some low temperature the pull of the thermostatis sufiicient to separate the armature from the poles of the magnet,whereupon the contacts are closed with a snap action and the motorstarted.

In Fig. 4 I have shown a modified form of my invention in which thethermostat does not generate sufficient force when cold to release thearmature and reclose the circuit. In other words, the motor upon beingdeenergized is not automatically restarted when it has cooled. By meansof a button 3 1, however, the thermostat may be depressed forcibly todisengage the armature from the magnet and reclose the contacts.Preferably however the button cannot be depressed far enough to hold thecontacts closed on overload so that the device is trip-free of thebutton.

In Fig. 5 I have shown a modified arrangement or the magnet providingfor an arc blowout effect as well as snap action. In this form of myinvention the magnet 38 which is in the form of a disk with a centralgroove 39 is provided with a facing member 40 made of a suitable circuitmaking and breaking contact material such as silver, this materialfilling the groove 39. The armature 4| on the thermostat 2 is providedwith a facing layer 63 of a contact material such as silver. Theengaging faces of the parts of the contacts :39 and 43 are approximatelyspherical, the point of contact being the center of the face of themagnet, i. e., between the poles of the magnet. With this arrangementthe flux between the poles of the magnet is at right angles to the arebetween the two contacts when they are separated and consequently thearc is blown to one side in the direction of the slot 39 and quicklyextinguished.

While I have shown the thermostat connected in the circuit of the motorso as to be heated by current passing directly through it, it will beunderstood that the thermostat may be heated by a resistance heaterplaced near it through which the motor current passes, thethermostat ifdesired not being included in the circuit. For purposes of the appendedclaims the term thermal element is therefore defined to include suitableheating means which may be a separate heating resistance or acombination of the two.

While I have shown a particular embodiment of my invention, it will beunderstood, of course, that I do not wish to be limited thereto sincemany modifications may be made, and I therefore contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The combination with a bimetallic thermostat, control means operatedby a movable portion of said thermostat, an armature of magneticmaterial secured to said movable portion of said thermostat, ablock-shaped permanent magnet magnetized with both poles on one facemounted adjacent said armature so as to cause said thei mostat tooperate with a snap action and layers of electrical contact material onthe adjacent surfaces of said armature and said magnet, said layershaving contact surfaces shaped to engage each other at a point betweenthe poles of said magnet.

2. The combination with a bimetallic thermostat, control means operatedby a movable portion of said thermostat, an armature of magneticmaterial secured to said movable portion of said thermostat, ablock-shaped permanent magnet magnetized with both poles on one facemounted adjacent said armature so as to cause said thermostat to operatewith a snap action, said magnet being provided with a groove extendingacross said face between said poles, a layer of electrical contactmaterial on said face filling said groove, and a layer of electricalcontact material on said armature, said layers of contact materialhaving contact surfaces shaped to engage each other at a point betweensaid poles.

3. The combination with a bimetallic thermostat, control means operatedby a movable portion of said thermostat, a magnetic snap operating meansincluding a block-shaped permanent magnet part magnetized with bothpoles on one face and an armature part of magnetic material, one of saidparts being secured to said movable portion of said thermostat and saidother part being mounted adjacent thereto so as to cause said movableportion of said thermostat to snap from one position to another inresponse to change in temperature, said magnet being made of an alloyhaving a high coercive force and being provided with a groove extendingacross said face between said poles, a layer of electrical contactmaterial on said face filling said groove, and a layer of electricalcontact material on said armature, said layers of contact materialhaving contact surfaces shaped to engage each other at a point betweensaid poles.

4. The combination with a" base member, a flexible switch arm on saidbase, alcontact on the free end of said arm, a bimetallic thermostathaving one end secured to said base, a contact on the free endof saidthermostat cooperating with the contact on said switch arm, a disk-'shaped permanent magnet on said base member adjacent the free end ofsaid thermostat, an armature of magnetic material for said magnetsecured to the free end of said thermostat, a layer of non-magneticmaterial over the face of said armature, and an adjustment cam arrangedto be turned to adjust the current setting of said thermostat.

5. The combination with a base member provided with a ventilatingaperture, a flexible arm on said base lying in said aperture, a contacton the free end of said arm, a bimetallic thermostat having one endsecured to said base, a movable contact on the free end of saidthermostat cooperating with the contact on said arm, a bracket on saidbase extending over said thermostat, a disk-shaped permanent magnet onsaid bracket adjacent the free end of said thermostat, said bracketbeing provided with a depression for said magnet and with a projectionsecuring said magnet in said depression, an armature of magneticmaterial for said magnet secured to the free end of said thermostat, alayer of nonmagnetic material over the face of said armature, anadjustment projection on said thermostat near its fixed end, andanadjustment cam arranged to be turned to engage said projection so as toadjust the setting of said thermostat.

6. The combination with a base member made of insulating material, aflexible switch arm on said base, a contact on the free end of said arm,a bracket made of non-magnetic material secured to said base, abimetallic thermostat having one end secured to said bracket, a movablecontact on the free end of said thermostat cooperating with the contacton said arm, a permanent magnet on said bracket adjacent the free end ofsaid thermostat, an armature of magnetic material for said magnetsecured to the free end of said thermostat, and a layer of non-magneticmaterial over the face of said armature.

'7. The combination with a base member made of insulating materialprovided with a ventilating aperture, a flexible switch arm on said baselying in said aperture, a contact on the free end of said arm, a bracketmade of non-magnetic material secured to said base, a bimetallicthermostat having one end secured to said bracket, a movable contact onthe free end of said thermostat cooperating with the contact on saidarm, a diskshaped permanent magnet on said bracket adjacent the free endof said thermostat, said bracket being provided with a depression forsaid magnet and with a projection securing said magnet in saiddepression, and an armature of magnetic material for said magnet securedto the free end of said thermostat.

8. The combination with a base member made of insulating materialprovided with a ventilating aperture, a fiexible switch arm on said baselying in said aperture, a contact on the free end of said arm, a bracketon said base extending over said thermostat, a bimetallic thermostathaving one end secured to said bracket, a movable contact on the freeend of said thermostat cooperating with the contact on said arm, adisk-shaped permanent magnet on said bracket adjacent the free end ofsaid thermostat, said bracket being provided with a depression for saidmagnet and with a projection securing said magnet in said depression, anarmature of magnetic material for said magnet secured to the free end ofsaid thermostat, a layer of non-magnetic material over the face of saidarmature, and means for adjusting said thermostat with relation to saidbracket.

EDWARD J. SCHAEER.

